The use of various sensors to measure gaseous and non-gaseous constituent parameters of fluids has become common in recent years. Such sensors may include electrodes and the like, and are often used to measure constituent parameters of fluids such as biochemical fluids (e.g. blood), natural or sewage water, ferments, laboratory fluids, and the like.
Fluid constituent parameters which are frequently measured by such sensors include partial pressure of a gas constituent (e.g. pO.sub.2 and pCO.sub.2), pH, concentrations of various ions (e.g. sodium, potassium, calcium, chloride and the like), and concentrations of various organic molecules such as sugars (e.g. glucose), hormones and enzymes.
Prior to an accurate measurement of a fluid constituent parameter, often it is necessary to calibrate a sensor, including its associated instrumentation. Calibration techniques generally involve bringing a sensor into communication with a reference (calibration) liquid having a known concentration of the constituent to be measured.
A calibration fluid may be prepared for immediate use, however, such preparation is time-consuming and requires skill and accuracy. Once a calibration fluid is prepared, care must be taken to prevent interaction of the fluid with the surrounding environment which might alter its make-up and result in faulty calibrations. Such alterations in the make-up of a calibration fluid may result from permeation, diffusion, chemical reaction, and the like.
Because of the time and skill involved in the preparation of a reference fluid, and as a result of the problems associated with maintaining a reference fluid in usable condition over time, sensors are frequently calibrated using pre-packaged calibration fluids.
Two examples of packaging arrangements for reference solutions are described in U.S. Pat. No. 4,340,457 to Kater. One package includes a pair of electrodes for use as a potassium ion sensor, mounted in openings along the length of a cylinder containing a reference fluid. The cylinder has plastic caps at each end to seal the package. After calibration of the sensor, the caps are removed from the cylinder, the fluid is drained, and the cylinder may be inserted into an extracorporeal blood loop for measuring the potassium ion concentration of a patient's blood.
Another package described in the Kater patent includes a stoppered vial containing a reference fluid. A pair of electrodes for use as a potassium ion sensor are mounted in a catheter which is inserted through the stopper bringing the electrodes into communication with the fluid in the vial. After calibration of the sensor, the catheter may be removed from the vial and inserted into a blood vessel for measuring the potassium ion concentration of a patient's blood.
Thus, there remains a need for a system having a calibrated medical sensor which permits calibration to be performed in both a simple and reliable manner.